function switchImg() {
    //轮播图
    document.getElementById('switch').onmouseover = function(ev) {
        var li = ev.target;
        if (li.toString().indexOf("LI") > -1) {
            var num = li.getAttribute('data');
            document.getElementById('big-banner').style.backgroundImage = "url(" + num + ".jpg)";
        }
    }

}
switchImg();
